joining large fg pieces

Need some advice please.  I have a fg raider box cover for a full size chev truck and want to use it on my smaller 1953 gmc half ton box.  I want to keep the factory lip around the edge of the cover but need to make it smaller.  I'm thinking I'm going to have to cut it in four pieces to make it shorter and narrower as well.  Not sure though how to go about joining the four pieces together.  Planning on using plywood on the box to make a frame or support for the pieces to sit on so that I can line them up properly to join them.  Is this possible?  What do I need for structual support if anything to fill the gaps.  What kind of fg route should I follow to fill the gaps and join the pieces back together?  Any help would be greatly appreciated.  Thanks.

This is what I would do. Doesnt mean it will work its just what I would do. After you cut your pieces. I would get some of the 3m fiberglass panel bonding adhesive. Heres a link of what im talking about should be able to get it at a local auto body shop, http://www3.3m.com/catalog/us/en001/auto_marine_aero/automotive_aftermarket/node_GSNPLCPK5Bbe/root_GST1T4S9TCgv/vroot_GSLPLPKL4Xge/gvel_6DTWTJQPBBgl/theme_us_aad_3_0/command_AbcPageHandler/output_html

 I would bond the panel you cut together with this product in the seams.  Your going to have to clamp this together some how. Once it dries though it will hold it together extremely well. I would then lay fiberglass on the bottom part of the box let this dry. Then I would take like 1x 4 or 1x 6 and I would put some resin down on the part you just fiberglassed lay your boards down on this area then lay glass over the boards, so you glass the boards into the box.   I would put at least 4-5 layers down over the boards, and probably 2 under. This should make it strong enough to keep everything together.

This is just my opinion though. Im assuming your talking about like a hard shell tonneau cover but im not positive on this.

Learning the trade one fiberglass creation at a time!